The invention relates to a process for producing fibreboard from wood fibres, in which comminuted wood particles in the form of wood chips are treated with saturated steam under the application of pressure and heat in order to reduce the binding strength of the timber binders, the wood chips treated in this manner are comminuted in a mill to obtain wood fibres, the saturated steam/fibre mixture is fed to a first separator or separator group to separate the wood fibres from the gaseous transport medium while undergoing drying and the admixing of a glue-like binder, and the separated wood fibres are, possibly following the renewed admixing of a gaseous transport medium and additional processing steps, such as bin storage, sifting and/or separation of the wood fibres from the transport medium in a second separator, fed to a moulding and pressing line for production of the fibreboard.
In contrast to pressed particle board, bonded fibreboard made of wood fibres exhibits considerably greater strength, greater homogeneity and improved workability.
According to the prior art, the wood fibres for the production of fibreboard are obtained by comminuting raw wood into wood chips in a hog.
Possibly following preheating, the wood chips are subsequently treated with saturated steam, where the saturated steam is at a pressure of between roughly 7 and 10 bar and a temperature of roughly 140 to 170.degree. C. This steaming process plasticises the natural timber binders, such as resins or lignin, thus reducing their binding strength. This steaming process also softens the bonded wood fibres.
Following this pretreatment, the wood chips are fiberised in a mill containing saturated steam and the actual fibres are obtained in the form of thin, fine threads.
The saturated steam/fibre mixture is then blown through pipes to a so-called flow dryer, where, under reduced treatment pressure if necessary, liquid glue is injected and the wet steam is mixed with hot air at a temperature of, for example, 170.degree. C.
By absorbing the steam components of the wet steam, the added air cools down relatively drastically, drying the wood fibres, this resulting in a steam/air mixture with a relative humidity of about 40% and a dew point of approx. 50.degree. C.
This mixture, which is relatively dry in comparison with the wet steam, is then fed to one or more cyclones in order to separate out the wood fibres. In the separator, the hot air is blown to the outside, while the heavier fibres are discharged through the bottom.
Following any necessary further processing steps, the fibres obtained in this way can now be fed to a moulding and pressing line where the fibreboard is produced by hardening the glue.
Further treatment steps for the wood fibres may include, for example, bin storage, sifting with the help of a sifter and renewed separation in a separator situated downstream of the sifter.
Due to the high volume flow of hot air required, the known production process is very energy-intensive, particularly since the air serving as transport medium is discharged into the open as exhaust air after the first separator, where it may be necessary to filter dust or toxic particles out of the exhaust air with the help of costly filter installations.
In this process, the exhaust gas cools down so drastically that it is no longer practical in most cases to utilise the energy of the remaining heat content.
For this reason, additional energy sources must be provided in the production process described, which are responsible, for example, for the preheating of the wood chips or the preheating of the sifter air.